This invention relates generally to ultrasonic inspection apparatus and more particularly to an ultrasonic apparatus for detecting small voids or air pockets inadvertently formed in cast explosive billets used in two piece shells.
For a number of years the U.S. Navy has had evidence that a dangerous situation may result from the presence of air pockets at certain locations in its projectiles. Specifically, air cavities within the explosive and air gaps between the explosive and aft end of a projectile are considered problems. Theoretical calculations show that the tremendous setback forces experienced by the explosive in a gun launch situation may generate sufficiently high temperatures in the vicinity of the cavities due to adiabatic heating to cause a premature initiation of the explosive. Recent firing test results with 5 inches/54 projectiles indicate that possibly a combination involving both an explosive base-to-shell air gap and the presence of voids within the explosive volume is a critical situation in gun launch.
These facts demonstrate a need for the nondestructive inspection of the Navy's cast explosives. Traditionally, the billets have been X-rayed for the presence of air cavities inadvertently formed by gases trapped in the highly viscous explosive mixture during the casting process. However, a non-destructive inspection technique that would be less operator dependent and potentially more reliable or less costly, and would also provide for the real-time detection of flaws is preferred. An investigation of the ultrasonic technique originated with these goals in mind.